1. Field of the Invention
The present invention concerns a coating material supply device for supplying a coating material at a predetermined flow rate to various types of coating machines such as an air atomizing spray gun, an airless atomizing spray gun or an electrostatic atomizing bell or disc type coating machine. More specifically, it relates to a coating material supply device suitable to a case of supplying, e.g., a two-component type coating material comprising a main agent and a curing agent therefor at a predetermined ratio to a coating machine or to a case of supplying coating material of different colors selectively to a coating machine, e.g., in multicolor coating.
2. Description of the Prior Art
In the coating operation, if the flow rate of a coating material supplied from a coating material source to a coating machine is fluctuated, the amount and the area of spraying the coating material may very to possibly cause unevenness in the coated layers. Accordingly, it is necessary to maintain the flow rate of the coating material supplied to the coating machine always constant.
In view of the above, in the conventional coating material supplying devices, a rotary pump used for supplying the coating material under pressure from a coating material supply source is driven at a constant number of rotation so as to supply a constant amount of coating material to the coating machine.
However, even if the rotary pump is driven at a constant number of rotation, the flow rate of the coating material may vary due to the change in the pressure loss at the suction port or discharge port of the rotary pump depending on the flowing state of the coating material, etc. and there has been a problem, e.g., in a two-component coating material that the main agent and the curing agent therefor can not be supplied at an accurate mixing ratio.
In a two-component type coating material, the main agent and the curing agent supplied separately from their respective reservoirs have to be mixed in a precisely determined ratio upon or just prior to the spraying from the coating machine. If the flow rate for the main agent or the curing agent varies to cause a delicate change in the mixing ratio, no uniform curing can be obtained for the coated layer thus result in unsatisfactory coating such as defective drying or development of crackings in the coated layers.
In view of the above, it has been attempted in the prior art to maintain an accurate flow rate for each of the main agent and the curing agent depending on the mixing ratio by measuring the flow rate for these agents supplied individually from their respective reservoirs by means of a rotary pump to the coating machine by flow meters disposed respectively to the flow channel for the main agent and that for the curing agent, thereby controlling the output from each of the rotary pumps based on the measured values.
However, since most of two-component coating materials are highly viscous as compared with usual paints, it is extremely difficult to accurately measure the flow rate by the flowmeter disposed in the flow channel for the main agent or the curing agent. In addition, there has been a problem that the viscous coating material adheres to the flowmeter thereby causing erroneous operation or failure. Thus, it has been extremely difficult to maintain the flow rate constant upon supplying the coating material to the coating machine.
In order to overcome such problems, use of a supersonic type flowmeter may be considered for contactless external measurement for the flow rate. However, the flowmeter of this kind is not practical for this purpose since it is extremely expensive and results in another problem of picking-up external noises to cause erroneous operation.
Further, use of a gear pump may be considered for supplying a highly viscous paint under pressure. However, there has been a problem that the viscous coating material adheres and clogs at the bearing portion of the gear pump during long time operation to often interrupt the rotation of the pump. In addition, in the case of of using a highly viscous paint, particularly, a metallic paint, the metal ingredient is ground by the gear pump failing to obtain uniform coating quality.
Further, in a car coating line where coating materials of multiple colors, e.g., from 30 to 60 kinds of different colors are coated while conducting color-change, since the flow rate of the coating material of each color supplied under pressure from each of the coating material reservoirs by each of the pumps has to be controlled uniformly, it is necessary to dispose a flowmeter for the coating material of each color, which remarkably increases the installation cost.
There have been proposed, for the related prior art, Japanese Patent Application Laying Open Nos. Sho 56-34988, Sho 60-48160, Sho 61-120660, Japanese Utility Model Publication No. Sho 60-17250, Japanese Utility Model Application Laying Open No. Sho 61-191146, etc.